A third-party app provider would like to have users provisioned via a service endpoint before...

To have users provisioned via a service endpoint before users access their app from Salesforce, the identity architect should recommend using a connected app with user provisioning flow. A connected app is a framework thatenables an external application to integrate with Salesforce using APIs and standard protocols. A user provisioning flow is a custom post-authentication process that can be used to create or update users in the external application using a service endpoint when users access the connected app from Salesforce. This approach can provide automatic user provisioning with limited changes to the third-party app. References: Connected Apps, User Provisioning for Connected Apps